The Benefits of Working Late

One of the benefits of working late on a Sunday evening is that you might see something interesting out your office window that you would otherwise not see. Here was my view at about 8:15 PM last night, looking northeast from the Texas A&M campus:

This cloud is probably not on your cloud identification chart, but it’s fairly common in these parts. It’s called an orphan anvil.

A little scientific explanation ought to spoil the mood nicely. When a thunderstorm develops, the buoyant updraft eventually hits a level when it stops being buoyant. For nice, big thunderstorms, that level is the tropopause. The ascending air stops ascending and spreads out as new ascending air comes along behind it.

The situation is similar to slowly pouring water on your kitchen floor. The water stops sinking and hits the floor, and gradually spreads out into a larger and larger puddle as more water comes along behind it.

This spread-out part of a thunderstorm is called the anvil. While the updraft part of a thunderstorm is typically at most a few kilometers in diameter, the anvil from a single thunderstorm can spread out as much as hundreds of kilometers in extreme cases.

Eventually all good thunderstorms must come to an end, and the thunderstorm rains out and dissipates. But the anvil, composed of tiny ice crystals, may float along and survive for hours. If there is wind shear, the anvil can be carried far from its (former) parent thunderstorm.

Aside from having its parent thunderstorm missing, an orphan anvil looks a lot like a regular anvil. It is often possible to see fall streaks of precipitation descending from the anvil. Portions of the bottom of the anvil may exhibit mammatus, the tongue-like downward bulges of cloud caused by cloudy air passing over very dry air. The edges of an orphan anvil are often blurry and indistinct, as the cloud has lost its influx of buoyant air and has stopped actively growing.

This particular orphan anvil was composed of the merged upper-level outflow from several parent thunderstorms that formed north of Houston, the last of which was near Plantersville at 7:00 PM. By 11:00 PM, the cloud was no longer noticeable in satellite photographs.

[Addendum: See that little convective cloud in the center background? A little while later, after it grew a little taller, it was apparent that it was closer than the orphan anvil. That anvil was way up there, and huge.]

Now that I’ve completely spoiled it, it’s time to get the mood back. Here’s how the orphan anvil looked at 8:25 PM, lit up by the setting sun: